(Health.com) -- Almost half of the meat and poultry sold at U.S. supermarkets and grocery stores contains a type of bacteria that is potentially harmful to humans, a new study estimates.

Researchers tested 136 packages of chicken, turkey, pork, and ground beef purchased at 26 grocery stores in five cities around the country, and found that 47 percent contained Staphylococcus aureus (S. aureus), a common cause of infection in people.

What's more, roughly half of the contaminated samples contained strains of the bacteria that were resistant to at least three antibiotics, such as penicillin and tetracycline. Some strains were resistant to a half dozen or more.

Although the high contamination rates may sound alarming, the threat these bacteria pose to humans is still unclear.

"We know that nearly half of our food supply's meat and poultry are contaminated with S. aureus, and more than half of those are multidrug resistant," says Lance B. Price, Ph.D., the senior author of the study, which was published Friday in the journal Clinical Infectious Diseases. "What we don't know [is] how often these transfer to people. We need more studies to quantify the public health impact."

S. aureus, and drug-resistant strains in particular, can cause serious infections and even death in humans. However, simple precautions including cooking meat thoroughly, washing hands after handling meat, and keeping raw meat separate from other foods to prevent cross-contamination are believed to neutralize the risk of infection, according to experts not involved in the research.

"Numerous studies of this type done in other countries...have generally come up with the same findings, that multidrug-resistant S. aureus are present in a variety of animal meats," says Pascal James Imperato, M.D., the dean of the School of Public Health at SUNY--Downstate Medical Center, in Brooklyn. "But, so far, no one has been able to draw a connection between the presence of those bacteria in meats and human illness."

Multidrug-resistant bacteria strains are "always a concern for humans," says M. Gabriela Bowden, Ph.D., a bacteria expert and assistant professor at the Texas A&M Health Science Center, in Houston. "But if you follow the hygiene rules that you would follow for Salmonella or E. coli, there shouldn't be a problem."

The meat, which was sold under 80 different brands, was purchased in Los Angeles; Chicago; Washington, D.C.; Fort Lauderdale; and Flagstaff, Ariz. The variety and number of S. aureus strains found on the samples suggest that the livestock themselves -- rather than contamination during processing and packaging -- are the source of the bacteria, the study notes.

Each year farmers and ranchers give millions of pounds of antibiotics to farm animals, most of them healthy, to make them grow faster and to prevent -- rather than treat -- diseases, says Price, the director of the Center for Food Microbiology and Environmental Health at the Translational Genomics Research Institute, a nonprofit organization in Flagstaff.

The combination of bacteria, antibiotics, and livestock living in close quarters creates the perfect environment for bacteria to thrive and mutate, which may explain the high levels of drug-resistant S. aureus seen in the study, he adds.

Virtually all (96 percent) of the S. aureus strains Price and his colleagues isolated had developed resistance to at least one antibiotic. Strains resistant to three or more antibiotics were found in 79 percent of turkey, 64 percent of pork, 35 percent of beef, and 26 percent of chicken samples.

"It's four different meats from four different animals in different geographical areas," Bowden says. "[S. aureus] may be more prevalent than we think."

Methicillin-resistant S. aureus (MRSA), which has been a particular menace to humans in hospitals and communities alike, was found in one package each of beef, turkey, and pork, though not chicken. This sample size wasn't large enough to arrive at an accurate estimate of its prevalence in meat nationwide, according to the study.

The Centers for Disease Control and Prevention, the Food and Drug Administration, and the U.S. Department of Agriculture currently monitor the country's meat supply for evidence of four major types of antibiotic-resistant bacteria (including Salmonella and E. coli). The study findings suggest that S. aureus should be screened for regularly as well, the researchers say.